Police crackdowns to cut the supply of illegal drugs by removing dealers and criminal overlords actually lead to rises in drug-related violence, gun crime and murder, according to an international study. A review of 20 years of research into drug enforcement has found that attempts to snuff out the trade in illegal substances have the opposite effect to that intended, by creating a power vacuum when drugs barons are imprisoned which is rapidly filled by competitors eager to fight each other for the newly-vacated territory.

Campaigners for the reform of drugs policy said the findings, which follow numerous studies showing that prohibition has failed to stop narcotics from becoming more plentiful, added to the pressure on governments to declare the "war" on the £200bn global illicit drugs industry over, and adopt a policy of controlled legalisation.

The study by the Canada-based International Centre for Science in Drug Policy (ICSDP) found that heavy-handed tactics, ranging from attempts by the American-sponsored Colombian armed forces to eradicate drug cartels to the arrest of dealers in Sydney, had led to increases in violence. Often, this violence is fuelled by criminals arming themselves to profit from price rises caused by seizures of drugs or the dismantling by police of dealing networks.

The assessment of 15 reports on the relationship between violence and drug enforcement, presented yesterday at an international conference in Liverpool, found that 87 per cent of studies reported that police seizures and arrests led directly to increased violence.

Dan Werb, co-author of the ICSDP document, said: "The convention has been that law-enforcement action to reduce the availability of drugs, thereby increasing drugs prices and decreasing supplies, also has the effect of reducing violence. Not only has prohibition been found to be ineffective with regard to price and supply; this study has also shown that it is accompanied by an increase in drug-related violence.

"Prohibition drives up the value of banned substances astronomically, creating lucrative markets and worldwide networks of organised crime. Unfortunately, the evidence suggests that any disruption of these markets through drug-law enforcement seems to have the perverse effect of creating more financial opportunities for organised crime groups, and gun violence often ensues."

The study, which highlights the drug-related violence gripping Mexico as an example of the vicious circle fuelled by crackdowns, said researchers in Florida had recorded a five-fold increase in violence and property crime linked to drug arrests. Another study of six US cities found that attempts to shut down crack markets led to increased homicide rates in four of them.

A six-year Australian investigation into drug dealing in Sydney found that the arrest of dealers and subsequent disputes between rivals had contributed to murders and a substantial rise in non-fatal shootings with handguns.

Campaigners for a regulated market in drugs said the study bolstered the argument for legalising drugs and introducing a sliding scale of controls, ranging from membership of coffee-shop style premises for the sale of cannabis to licensed pharmacies selling cocaine.

A spokesman for the Transform Drug Policy Foundation said: "We have a government in pathological denial of the negative impact of a prohibition-based drugs culture. Which other global industry worth £200bn is left in the hands of organised criminals rather than being taxed and properly regulated?"